Semiconductor fabrication commonly requires polishing of a wafer. Machines for preparing and fabricating semiconductor wafers are known in the art. Wafer preparation includes slicing semiconductor crystals into thin sheets, and polishing the sliced wafers to free them of surface irregularities to achieve a planar surface. Typically, it is necessary for the formation of various circuits or for other uses of wafers, that the active or front face, e.g., the face of the wafer on which the integrated circuitry is to be formed, be highly polished.
In general, the polishing is accomplished in at least two steps. The first step is a rough polishing or abrasion. This step may be performed by an abrasive slurry lapping process in which a wafer mounted on a rotating carrier is brought into contact with a rotating polishing pad upon which is sprayed a slurry of insoluble abrasive particles suspended in a liquid. Material is removed from the wafer by the mechanical buffing action of the slurry. The second step is fine polishing. The fine polishing step is performed in a similar manner to the abrasion step, however, a slurry containing less abrasive particles is used. Alternatively, a polishing pad made of a less abrasive material may be used. The fine polishing step often includes a chemical mechanical polishing ("CMP") process. CMP is a combination of mechanical and chemical abrasion, and may be performed with an acidic or basic slurry. Material is removed from the wafer due to both the mechanical buffing and the action of the acid or base. Such polishing is also important during the manufacturing of semiconductor devices in order to planarize various thin film layers formed on the surface of a semiconductor wafer. The thin film may, for example, be an interlayer insulating film formed between two metal layers, a metal layer, or an organic layer.
Usually, polishing apparatuses bring the face of the wafer to be polished into engagement with a treating surface, such as the polishing surface of a rotating polishing pad having a desired polishing material, e.g., a slurry of colloida silica, applied thereto. In many instances, the polishing head which holds the wafer with the face exposed also rotates. It is the movement between the wafer and the polishing pad which results in the desired polishing. In some instances, polishing is provided primarily to make one face flat, or parallel to another face.
A polishing apparatus is shown in FIG. 1. A wafer 10 is held in a wafer carrier 20 by a guide ring 30. Optionally, a backing film 40 can be inserted between the wafer 10 and the carrier 20. The backing film 40 in combination with the guide ring 30 minimizes vertical movement of the wafer 10 during polishing. Without the backing film 40, the wafer 10 can freely move in the vertical direction during polishing. A backing film 40 or insert pad has been used in the wafer carrier 20 to keep the wafer 10 in contact with the surface of the polishing pad 50 to improve polished wafer surface uniformity.
The polishing pad 50 is affixed to a polishing table 60. In FIG. 1, the polishing table 60 is rotatable about its central axis 65. Wafer carrier 20 is also rotatable about its central axis 25, which except for a limited oscillating motion relative to the polishing table 60, is fixed relative to the central axis 65 of the polishing table 60. In operation, the polishing table 60 rotates at a first predetermined speed about its central axis 65, thereby presenting a continuously advancing polishing surface, i.e., polishing pad 50, to the layer being planarized. While wafer carrier 20 rotates at a second predetermined speed about its central axis 25, the wafer 10 is polished along an annular polishing area of the polishing table 60.
The polishing process is conducted by placing the wafer 10 within the cavity formed in the wafer carrier 20 by the backing film 40 and guide ring 30 so that wafer 10 contacts the polishing pad 50. During polishing, polishing pad 50 is supplied with an aqueous slurry 70 via supply nozzle 80, while the polishing table 60 rotates about its central axis 65. The materials of the polishing table 60, wafer carrier 20 and slurry 70 should be non-contaminating and, except for polishing action, non-destructive to the wafer 10 being polished.
At the inner portion of the wafer 10, apart from the edge of the wafer 10, the surface being polished is in continuous contact with the polishing pad 50. Therefore, the pressure applied by the polishing pad 50 across the inner portion of the wafer 10 is nearly constant. In contrast, the edge of wafer 10 constitutes a border between the area where the wafer 10 is in contact with the polishing pad 50 and the area where it is not in contact. Thus, at the outer portion of the wafer 10 including the edge, there is an irregularity of pressure applied during polishing. This results in nonuniform removal of the material from the wafer. Consequently, a portion of the surface of the wafer 10 may become overpolished or underpolished. When a backing film 40 is not used with the wafer carrier 20, the irregularity in the polished surface is relatively small since the wafer 10 can move freely during polishing. However, when a backing film 40 is used as shown in FIG. 1, the wafer 10 is fixed and the irregularity in the polished surface is more substantial. Thus, a backing film enhances surface uniformity at least with respect to all portions of the wafer, but for the edge. Uniformity between the edge and inner portion of a wafer is better served by not using a backing film.
Overpolishing causes the material being polished to become thinner which can adversely affect the performance and reliability of the semiconductor devices on the wafer. If a portion of the wafer is underpolished, the underpolished layer of material will likely be insufficiently planarized or remain too thick. Thus, subsequent electrical contact processing may not completely provide sufficient contact or sufficient insulation resulting in the formation of undesirable electrical open circuits or undesirable short circuit paths.